It’s a commonly perceived myth that aquarium animals should be kept individually. People often say “You shouldn’t keep more than one of (these) per tank.”
We have learned small animals like invertebrates (shrimp, crabs, snails) actually thrive in groups. In nature animals are always surrounded by an array of life, they see others like themselves and feel safe. A sense of ease comes from having a friend. Don’t believe everything you hear at your LFS, do your own research!
Almost every animal variety we keep is housed with others like them. Sparring/Play-fighting is normal for most animals. They will size each other up and from time to time bickering can happen between even the best of friends. But in general, most species do better in pairs or groups.
Some exceptions exist, and not all species are groupable. Ultimately many factors play a roll such as tank size, animal age/size, other tank mates, feeding schedule, etc.
In general…
Shrimp are collected in groups, crabs always have an amigo beyond the next rock, and most snails and mollusks are practically blind (so they won’t barely even notice others). In most cases you should feel safe buying a friend or two for your pets.